every so often my 99 xj8 will go into restricted performance and will throw rich codes right and left, then will turn off. any ideas whats happening?

 

Post the actual codes here to give everyone a better chance to see exactly what's happening and give appropriate advice.

Graham

 

p0172, p0174

 

That's usually a vacuum leak in the intake trunking or the part load breather hose. It's a pain sometimes to find. Your's sounds pretty big though. Take all the covers off and look around and listen. It might be obvious.